Methods are known consisting of placing a series of parallel submerged seismic cables (or lines or streamers), on each of which sensors of the hydrophone and/or geophone type are placed spaced apart, the cables being pulled by one or more boats.
One (or more) other boat(s), called “source” boats, provided with means capable of creating a wave in a sea medium, generally in the form of an air gun, move at a distance from the sensor cables. The winds thus formed spread as far as the sea bottom, then on the different geological layers to be reflected by the latter, and are lastly collected and measured by said submerged sensors. The source boat may be the boat pulling the seismic cables.
All of the information is then processed to produce a three-dimensional (3-D) image of the different geological layers of the underwater subsoil, generally used to determine the presence of any oil-bearing reservoirs.
This technique has been used for many years and is subject to very restrictive implementation requirements. First, the dynamic noise due to towing of the cables disrupts the measurement of the waves one seeks to collect. Furthermore, the hydrodynamic drag resulting from the drag of the cables is very high, and can be counted in dozens of tons, for example approximately 70 tons, which leads to the use of very powerful pulling boats. This is due in particular to the speed required in the water for the method in the presence of paravanes, which create resistance. Furthermore, the weight and the hydrodynamic drag caused make the pulling cable of the paravanes undergo a dynamic deformation effect of the “piano wire” type during towing. This leads to fatigue of the cable and may cause it to break. This may result in extremely high replacement costs, given the immobilization of the entire device. Furthermore, in the traditional methods, the cables must be weakly submerged, between 5 and 10 m, which causes an accident risk given the circulation of vessels on the surface with a strong draught (oil tankers or container ships) and high sensitivity to the condition of the sea.
Furthermore, the known seismic prospecting devices leave shadow regions during measurement. In fact, the cables generally have a length of up approximately 8 km and are spaced approximately 100 m apart, which leads, for a dozen parallel cables, to a measuring area of 1×8 km. However, the ideal in terms of measurements is to use an isotropic system, i.e. a square surface, for example 8×8 km. However, these dimensions are incompatible with the towing means that would be necessary in light of the weight, drag, and logistics necessary to obtain such a measuring surface.
Efforts have therefore been made to resolve the situation in two known manners.
The first attempt (called Wide Azimuth) consists of making up for the anisotropy, by using one (or two) boats pulling a set of cables forming a measuring area of 1×8 km, and using 2 to 8 source boats. This device has two major drawbacks. First, the prohibitive cost resulting from the investment material, maintenance and use (2 to 8 source boats, plus one (or two) towing boats, plus all of the cables). The other drawback lies in the fact that the source boats “fire” (i.e. emit waves) each in turn, and therefore 2 to 8 times less frequently, which leads to a very low firing density.
The second attempt proposed in a known manner is shown by patent application GB no. 2,435,931, in the name of Western Geco, which describes a method and device diagrammatically consisting of an array of sensors (geophones) fastened to a two-dimensional structure (in the form of a mesh or net) or three-dimensional structure. This structure has a periphery (perimeter or enclosure) kept in shape by dynamic means such as drones or small boats, so as to maintain the shape of the mesh making up the structure. The latter is continuously pulled and one or more seismic sources are provided.
Despite the apparent draw, theoretically speaking, of the device and method thus proposed, the fact remains that this device appears difficult to implement realistically. In fact, the structure thus formed would have an enormous weight and drag and would require the use of means to maintain the shape that are disproportionate and unmanageable both technically and financially or in terms of budget. Furthermore, due to its construction, it only offers a single possible geometry for the array of sensors.
According to another aspect, generally, marine seismic prospecting aims to sense or recover the maximum quantity of signals to perform the most accurate and reliable possible geographical mapping of the underlying areas of the seabed. However, low-frequency signals provide information on very deep reservoirs and are therefore precious in that respect. Low-frequency signals are, however, greatly attenuated by the surface reflection phenomenon, called “phantom,” and owing in particular to the fact that the cable, according to the prior art, is submerged several meters from the surface. Efforts are thus made to eliminate these “phantoms” to obtain what is called a “flat spectrum.” Attempts have been made to resolve the situation by using a technique known as “over-under” that consists of positioning two cables bearing hydrophone sensors, one under the other vertically, at respective depths for example of 20 m and 26 m. The processed combination of the two signals received by the two respective cables makes it possible to attenuate or eliminate the consequences of “phantoms.” However, this known method, aside from the additional processing it requires, has the major drawback of very greatly decreasing productivity and increasing costs, due to the doubling of the cables and sensors.
Another known techniques seeking to eliminate “phantoms,” proposed by the company PGS, consists of using lines or cables bearing, in addition to the hydrophones (measuring the pressure), geophones or accelerometers capable of measuring the speed or acceleration of the wave. Reflection coefficients for the respective pressure (hydrophones) and speed (geophones) measurements being inverses (−1 and +1), it is thus theoretically possible to cancel out the “phantoms.” This known technique has the drawbacks of requiring a high investment in terms of sensors and creating bothersome noise at the geophones or accelerometers resulting from the pulling speed (approximately 5 knots) generating parasitic vibrations.
The invention proposes to resolve at least some of the aforementioned drawbacks.